A Long-Term Evolution (LTE) system offers high peak data rates, low latency, improved system capacity, and low operating cost resulting from simplified network architecture. LTE systems also provide seamless integration to older wireless network, such as GSM, CDMA and Universal Mobile Telecommunication System (UMTS). In LTE systems, an evolved universal terrestrial radio access network (E-UTRAN) includes a plurality of base stations, such as evolved Node-B's (eNBs) communicating with a plurality of mobile stations referred as user equipment (UEs). Dual Connectivity (DC) UE is introduced to enhance bandwidth and flexibility use of the network. A UE with dual connectivity has more than one transceivers corresponding to more than one MAC entities. The multiple MAC entities can be configured to communicate with multiple eNBs simultaneously.
In LTE Rel-10, the concept of carrier aggregation (CA) has been introduced to enhance the system throughput. With CA, two or more CCs are aggregated to support wider transmission bandwidth up to 100 MHz. A Rel-10 UE with reception and/or transmission capabilities for CA can simultaneously receive and/or transmit on multiple CCs corresponding to multiple serving cells. When CA is configured, the UE has only one RRC connection with the network. At RRC connection establishment/reestablishment or handover, one serving cell provides the NAS mobility information. At RRC connection reestablishment or handover, one serving cell provides the security input. This cell is referred to as the primary serving cell (PCELL), and other cells are referred to as the secondary serving cells (SCELLs). Depending on UE capabilities, SCELLs can be configured to form together with the PCELL as a set of serving cells.
The demand for higher bandwidth may require exploiting further on CA operation to aggregate cells from different base stations to serve a single UE, called inter-eNB carrier aggregation (inter-eNB CA). Inter-eNB CA not only can provide enhanced throughput, it offers other benefits such as spatial diversity (or so-called multi-site diversity) gain and reduction of mobility management overhead in heterogeneous networks (HetNet). In LTE Rel-12 and after, besides the macro eNBs, small eNBs with low transmission power and simplified protocol stacks or functionalities are introduced into E-UTRAN, which is referred to as HetNet smallcell networks. The smallcell architecture can enhance the data throughput and reduce the mobility signaling overhead.
In dual connectivity, a UE is simultaneously connected to a master eNB (MeNB) and a secondary eNB (SeNB) in a HetNet smallcell network. Radio link monitoring (RLM) and radio link failure (RLF) detection is applied on PCELL associated with the MeNB. Upon RLF, UE performs RRC connection re-establishment, in which the first step is to search for a suitable cell via cell selection. While the UE may intuitively try to re-establish RRC connection on the macrocell layer, for re-establishment due to RLF-related causes, it is likely that all suitable cells are associated with the SeNB.
The RRC re-establishment can succeed only when performed on an eNB with valid UE context, and such eNB is said to be a “prepared” eNB. Since a SeNB is not fully prepared, re-establishment on the SeNB is highly probable to fail. However, the re-establishment can be successfully completed if the SeNB has properly obtained the UE context. It is desirable to improve the re-establishment success rate for SeNB. Furthermore, it is desirable to only select a preferred SeNB for re-establishment based on smallcell HetNet and UE conditions.